For exhaust emission control, catalysts are used in the exhaust systems of both spark-ignition engines and diesel engines. In the case of spark-ignition engines, these catalysts consist of ceramic filters which are coated with catalytic materials, for example platinum powder. As a result, the engines emit fewer pollutants in their exhaust gases, such as carbon monoxide, unburnt hydrocarbons and nitrogen oxides. The most common exhaust catalysts are so-called three-way catalysts, whereby NO is reduced to N.sub.2, CO is oxidized to CO.sub.2 by the oxygen from the NO, and the hydrocarbons are oxidized. These catalysts operate at lambda values, that is to say air excess values, of 1.+-.0.02. A lambda probe is required for adjusting the mixture corresponding to this value.
Similar catalysts are used for diesel engines, together with ceramic or metallic support materials. In the diesel engine, however, the problem is a pure oxidation, because a reduction is impossible due to the air excess. In the diesel engine, a catalyst therefore has the task of burning carbon monoxide, hydrocarbons and soot. In addition, soot particle filters are used in diesel engines in order to reduce soot impingement, and these filters can also be catalytically coated, in order to reduce the ignition temperature of the collected soot and hence to burn the soot particles. Filter regeneration can then be achieved in this way. At the same time, this also has a favorable influence on the exhaust emission.
If a spark-ignition engine is supercharged by a gas-dynamic pressure-wave supercharger, the three-way catalyst must be placed on the high-pressure side since, due to the flushing of the cellular wheel of the pressure-wave supercharger, the air excess in the low-pressure exhaust can reach very high values. Consequently, the condition of a lambda value of 1.+-.0.02 cannot be met in the low-pressure side. Furthermore, if the catalyst is arranged in the low-pressure exhaust, the back pressure of the latter rises, so that adequate flushing of the cells of the cellular wheel would not be ensured.